Microstructure and mechanical properties of IF/St52 steel composite produced by friction stir lap welding

In the present research, a new composite including interstitial free (IF) steel as the outer layer and St52 as the inner layer was fabricated by friction stir lap welding (FSLW). The microstructures and mechanical properties of initial and composite samples were investigated. It was found that the quality of interfacial bonding for the tool traverse speeds of 70 and 100 mm/min was excellent due to the higher strain rate. It was observed that the microstructure of composite samples consisted of fine equiaxed ferrite, Widmanstatten ferrite, martensite, grain boundary ferrite, and aggregates of ferrite + cementite. The increase of tool traverse speed from 40 to 70 and 100 mm/min at a constant tool rotation speed of 1000 rpm decreased the cooling time leading to a higher cooling rate and the formation of martensite phase. The grain size of the composite produced by the traverse speed of 40 mm/min in the first and second HAZ regions was larger than that of the 70 mm/min due to the higher heat input and therefore lower cooling rate of the sample welded by 40 nun/min. The hardness of the center of the stirred zone (SZ) was higher than that of the advancing and retreating sides. The St52 layer of composites showed the microhardness value of similar to 190 HV whereas the initial St52 sheet had a microhardness around 160 HV. The composite produced by the traverse speed of 70 mm/min had the highest yield and ultimate tensile strength. This high strength achieved without a remarkable decrease in elongation due to the formation of a heterogeneous microstructure. The fracture surfaces indicated that the bonding between IF and St52 steels in the composites produced by traverse speeds of 70 and 100 mm/min was high enough strong to maintain integrity up to a high level of deformation during the tensile test. Finally, with increasing the tool traverse speed, the size and depth of dimples decreased.